1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic system and, for example, to an image forming apparatus such as a copier, a printer, a facsimile machine and the like.
2. Related Background Art
FIG. 8 is a schematic sectional view of a conventional color image forming apparatus of electrophotographic type employing an intermediate transfer member. In such an image forming apparatus, the step of primarily transferring a toner image formed on a photosensitive member to the intermediate transfer member is repeated for a plurality of color toners, and the toner images on the intermediate transfer member in a secondary transfer step are collectively transferred to a transfer material such as a paper, so that a full color image can be obtained.
Now, an image forming process will be described.
The surface of a photosensitive member 101 as an image bearing member is uniformly charged by a primary charger 107, and an image pattern corresponding to an original image is exposed by a laser exposure apparatus 105 to form an electrostatic latent image on the photosensitive member 101. At the time of developing, a developing bias is applied to the developing sleeves of developing devices 108, 109 to develope the electrostatic latent image on the photosensitive member into a toner image. A transfer bias is applied from a primary transfer roller 106 to an intermediate transfer member 102, and in an primary transfer nip portion, the toner image on the photosensitive member is transferred to the intermediate transfer member 102 by an electric potential difference between the photosensitive member 101 and the intermediate transfer member 102. By repeating a series of the above described steps for four color toners of yellow (Y), magenta (M), cyan(C) and black (K), a full color image is formed wherein toner images of four different colors are superimposed on top of each other on the intermediate transfer member 102. Note that, after the primary transfer, the toner remaining on the photosensitive member is removed by a cleaning blade 110 and collected in a cleaning container.
Next, a secondary transfer belt 103 separated from the intermediate transfer member 102 during a series of the above described steps is brought into pressure contact with the intermediate transfer member 102, and when a paper or the like as a transfer member Pxe2x80x2 is passed through between the secondary transfer belt 103 and the intermediate transfer member 102, a full color toner image is transferred from the intermediate transfer member 102 to the transfer member. Thereafter, the transfer member is conveyed to a fixing device in which the toner image is subjected to color mixing and at the same time fixed to the transfer member, thereby obtaining a full color image as a permanent image.
The secondary transfer residual toner remaining on the intermediate transfer member 102 is removed by a cleaning member (a blade) 104 and collected in a cleaning container.
However, in the above described configuration, there have been problems as follows.
Since a charged state of the photosensitive member and a charged amount per unit weight of the toner vary with the durability thereof and circumstances, an image density also varies with such a fluctuation. In order to avoid this, the method was employed wherein an image for detection (hereinafter referred to as a patch image) responding to a definite density signal is formed on the photosensitive member and the image density thereof is detected by a sensor and, on the basis of the detected signal, a charging bias inputted to the primary charger 107, an exposure intensity by the exposure device 105 and the charged amount per unit weight of the toner are controlled. There has been known the method wherein the density of the above described patch image is detected on the photosensitive member or transferred to the intermediate transfer member and detected on the intermediate transfer member. Particularly in recent years, in keeping with the miniaturization of the apparatus, it is becoming difficult to obtain a space (a space for installing a sensor) for detecting the density on the photosensitive member and there are many cases where the method for detecting on the intermediate transfer member is adapted. However, in this case, since the density of the patch image formed on the photosensitive member is not directly detected, but detected after it is once transferred to the intermediate transfer member, a transfer efficiency of the patch image should be very high.
A transfer bias (a transfer voltage or a transfer electric current) inputted to the primary transfer roller 106 which is set at the time when a toner image of plural colors is multi-transferred from the photosensitive member to the intermediate transfer member is determined by several conditions. Among them, because a transfer efficiency and a re-transfer rate greatly contribute to density, hue or tone and the like, they are highly valued. FIG. 2 is a graph showing a relationship between a transfer residual rate and a transfer electric current and a relationship between a re-transfer rate and the transfer electric current. The continuous line A represents the transfer residual rate. The broken line B represents the re-transfer rate. As shown in FIG. 2, when the above described transfer electric current is increased, the transfer efficiency (the primary transfer efficiency) is raised (a transfer residual rate A is decreased). However, the re-transfer rate B is also raised. Therefore, at the time when the transfer bias is set, both the transfer efficiency and the re-transfer rate may be preferably optimized.
Note that the transfer efficiency is represented by the proportion of the toner density (the transfer residual rate is represented by the proportion of the transfer residual toner density on the photosensitive member) on the intermediate transfer belt at the time when the sum of the toner density on the intermediate transfer belt subsequent to the primary transfer and the transfer residual toner density on the photosensitive drum is represented by 100. If the toner amount(density) of the toner image formed on the photosensitive member is represented by X and the toner amount (density) of the toner image transferred to the intermediate transfer member with this toner image on the photosensitive member transferred to the intermediate transfer member is represented by Y, the following expression is established:
(Y/X)xc3x97100(%).
Also, the transfer residual rate (which means the rate wherein a toner image is not transferred to the intermediate transfer member but has remaining on the photosensitive member and is contrary to the above described transfer efficiency) is represented by the following expression:
{(Xxe2x88x92Y)/X}xc3x97100(%).
Also, the re-transfer rate is represented by the proportion of the density of the toner re-transferred to the photosensitive member at the time when the sum of the density of the toner image transferred to the intermediate transfer member and the density of the toner re-transferred (offset) to the photosensitive member when the toner image on the intermediate transfer member passes through the primary transfer portion for the next time is expressed by 100. The re-transfer rate is represented by the following expression if the amount of the toner re-transferred to the photosensitive side is expressed by Z:
(Z/Y)xc3x97100(%).
The above described transfer efficiency, transfer residual rate and re-transfer rate are derived from measuring by densitometer (a product name: 404, manufactured by X-rite Corporation) the density of each toner removed by Mylar tape and attached to CLC80 g/m2 paper adapted by us as the standard paper for color.
However, in the case where a Dmax patch image (the Dmax referred to herein means the maximum density and the Dmax patch means a developed patch image as against the latent image which is Dmax as an image signal) on the photosensitive member used for controlling and adjusting the charged state on the photosensitive member and the charged amount per unit weight of the toner is transferred to the intermediate transfer member and the density of the Dmax patch image is detected on the intermediate transfer member, the transfer bias value set as described above will create problems in the following respects. As the charged amount of the toner per unit weight and the charged state of the photosensitive member suddenly fluctuate according to changes in durability and circumstances, if the weight per unit area (hereinafter referred to as a bearing amount) of the toner image formed on the intermediate transfer member exceeds a predetermined amount (the maximum bearing amount when formed on the transfer member), the transfer bias value set as described above is unable to transfer the Dmax patch image on the photosensitive member sufficiently to the intermediate transfer member, thereby causing a transfer deficiency. In such a state, when the Dmax patch image on the intermediate transfer member is detected by an image density detecting sensor, it is detected lower than the density of the Dmax patch image formed on the photosensitive member and, therefore, it is impossible to adequately control and correct the density of the toner image formed on the photosensitive member. This is because the normal transfer bias value set as described above is set in consideration of not only the transfer efficiency but also re-transferring.
Moreover, in the image forming apparatus wherein the residual toner remaining on the photosensitive member subsequent to the primary transfer is electrostaticly collected by a developing device without being scrubbed and removed by the conventional cleaning blade 110, if the amount of the above described toner residues is high, a phenomenon appearing as a ghost image in the next several cycles of the photosensitive member is developed, thereby causing serious problems. However, the relationship between the transfer residual rate and the transfer bias (the transfer bias applied to the primary transfer roller 106) and the relationship between the re-transfer rate and the transfer bias are, as shown in FIG. 2, in a relationship of trade-off. Therefore, in consideration of the balance between both of them, if the amount of the above described residual toner is set so as to be at the most smallest level as the transfer bias value, the transfer efficiency does not come to the maximum value (the transfer residual rate does not come to the smallest value), and similarly the re-transfer rate does not come to the smallest value.
Also, even in the image forming apparatus wherein the toner image of each color on the photosensitive member is multi-transferred to the transfer member which is borne by a transfer belt as a transfer material bearing member, the problems similar to the above have occurred.
An object of the present invention is to provide an image forming apparatus capable of preventing the deterioration of the efficiency of transferring the image from the image bearing member to the intermediate transfer member in a second mode.
Another object of the present invention is to provide an image forming apparatus for preventing the deterioration of the efficiency of transferring the image from the image bearing member to the transfer material bearing member in a second mode.
Still another object of the present invention will be apparent by reading the following detailed description.